The present invention generally relates to devices for performing standard electrospray ionization and nanoelectrospray ionization.
Electrospray ionization is used to transform a liquid sample into gaseous ions. A sample solution is forced or pulled through a small sprayer needle so that a fine mist of nebulized sample droplets is created. The sample is sprayed toward a counter-electrode with a high voltage applied between the solution and the counter-electrode. The high voltage causes charged molecules to be formed from the solution.
One application of electrospray ionization has been the formation of ions from an analyte sample for analysis by mass spectrometry, which can produce an analysis based on very few molecules. A sample is typically sprayed at a source orifice of a mass spectrometer with high voltage applied between the solution and the orifice to generate the ions for analysis. Because of the importance of analyzing small amounts of biological samples (particularly complex biological samples), a great deal of interest has arisen in the use of low flow rate electrospray ionization devices.
Nanoelectrospray ionization is a subset of the electrospray ionization technique that uses very low flow rates to allow the analysis of very small amounts of sample by mass spectrometry. Common volumetric flow rates for nanoelectrospray ionization are in the nL/min range. In order to achieve a stable electrospray at such low flow rates, very small sprayers must be used. Typical sprayer needles used with nanoelectrospray ionization have openings with diameters in the 1-75 xcexcm range, whereas standard electrospray ionization sprayer needles usually have openings of 75-300 xcexcm in diameter. Such nanoelectrospray needles are fabricated using special techniques, usually by melting and pulling a larger capillary down to a smaller opening. In order to prevent sample carryover between experiments and because the tips of the nanoelectrospray needles are very fragile, the needles are usually only used for a single sample and are then discarded.
Capturing the entire plume of ions created with standard electrospray ionization is difficult because the plume can be several centimeters in diameter and the inlet orifice (e.g., a transfer capillary) on the vacuum system of the mass spectrometer is typically less than a millimeter in diameter. Any portion of the electrospray ionization plume not captured by the transfer capillary is wasted sample. One solution to this problem is U.S. Pat. No. 6,107,628 to Smith et al., which describes an apparatus for directing ions generated at atmospheric pressures into a region under vacuum. The apparatus of the ""628 patent comprises a plurality of elements contained within a region maintained at a pressure between 10xe2x88x921 Millbrae and 1 bar, each of the elements having progressively larger apertures to form an xe2x80x9cion funnelxe2x80x9d having an entry at the largest aperture and an exit at the smallest aperture. An RF voltage is applied to each of the elements so that the RF voltage applied to each of the elements is out of phase with the RF voltage applied to the adjacent element or elements. Although the apparatus of the ""628 may achieve the goal of focusing a dispersion of charged particles, it does so by complicating the design of the electrospray ionization source.
In nanoelectrospray ionization, the small aperture size of the nanoelectrospray needles reduces the applied voltage necessary to sustain a spray, and the sprayer needle is thus positioned much closer to the sampling orifice than in electrospray. As a result of the shorter distance between the sprayer needle and orifice, and because of the smaller diameter sprayer needle, the plume from nanoelectrospray ionization is much smaller in size than the plume from standard electrospray ionization and most, if not all, of the ions created by nanoelectrospray ionization may be captured by the transfer capillary and sent to the mass spectrometer for analysis. This increase in efficiency is one of the main reasons nanoelectrospray ionization produces higher sensitivity than standard electrospray ionization. However, in order to have most or all of the ions that are created transferred into the mass spectrometer, the nanoelectrospray ionization needle must be precisely aligned with the small orifice into the mass spectrometer vacuum system. This alignment is difficult and is often only achieved using complicated and expensive cameras and microscope lenses. Additionally, because the nanoelectrospray needles are not commonly re-used (as is the case with standard electrospray ionization needles), the alignment has to be performed for every sample to be analyzed.
Researchers at Bruker Daltonics Inc. recently proposed a zero adjustment device for nanospray mass spectrometry as a solution to this problem. (See Wang et al., xe2x80x9cZero Adjustment Device for Nanospray Mass Spectrometryxe2x80x9d, Proceedings of the 48th ASMS Conference on Mass Spectrometry and Allied Topics, Long Beach, Calif., 2000; pp. 379-380.) The zero adjustment device is a sub-unit that can be detached from an electrospray ionization source for the sample loading, nanospray needle exchanging, and source cleaning. A pre-opened nanospray needle is self-aligned by a needle mounting union and is inserted into an ionization channel when mounted. The needle position is fixed and no fine adjustment is needed. The ionization channel is attached to a pre-capillary used as an interface between the ionization channel and the main electrospray ionization capillary. The zero adjustment nanospray device can be operated with the needle tip in a wide range of positions, which allows more tolerances on spray needle mounting. However, neither the construction of the metal ionization channel used in the zero adjustment nanospray device nor the task of interfacing the zero adjustment nanospray device with different source designs are simple tasks.
Internal calibration of a mass spectrum produces the most accurate peak assignments of an analyte solution because the calibration ions experience essentially the same conditions as the analyte ions. Typically, a calibration solution is added to the analyte solution before it is electrosprayed. However, when electrospraying two solutions containing ions of interest, ionization suppression can occur. Ionization suppression occurs when one of the species present (i.e., either the analyte or the calibrant) is more easily ionized, thereby effectively suppressing the signal of the other species contained in the sample. In addition, mixing two solutions with different solvent systems can cause problems with adduct formation, solubility, and/or reactivity.
In order to try to avoid ion suppression and other problems occurring when electrospraying mixed solutions for internal calibration of a mass spectrum, multiple sprayer standard electrospray ionization has been proposed. The analyte solution is loaded into one of the electrospray ionization needles while a calibration solution is loaded in another. The needles are either aimed at a single sampling orifice or separate orifices are used and the streams are mixed once inside the vacuum system of the mass spectrometer. The use of two or more spray needles with standard electrospray ionization sources has been demonstrated by several research groups. (See, e.g., Andrien et al., xe2x80x9cMultiple Inlet Probes for Electrospray and APCI Sourcesxe2x80x9d, Proceedings of the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Fla., 1998; p. 889.; Dresch et al., xe2x80x9cAccurate Mass Measurements with a High Resolution Dual-Electrospray Time-of-Flight Mass Spectrometerxe2x80x9d, Proceedings of the 47th ASMS Conference on Mass Spectrometry and Allied Topics, Dallas, Tex., 1999; p. 1865-1866.; Jiang et al., xe2x80x9cDevelopment of Multi-ESI-Sprayer, Multi-Atmospheric-Pressure-Inlet Mass Spectrometry and Its Application to Accurate Mass Measurement Using Time-of-Flight Mass Spectrometryxe2x80x9d, Anal. Chem. 2000, 72, 20-24; Hannis et al., xe2x80x9cA Dual Electrospray Ionization Source Combined With Hexapole Accumulation to Achieve High Mass Accuracy of Biopolymers in Fourier Transform Ion Cyclotron Resonance Mass Spectrometryxe2x80x9d, J. Am. Soc. Mass Spectrom., 2000, 11, pp. 876-883.)
Although dual sprayer standard electrospray ionization has been demonstrated, dual sprayer nanoelectrospray ionization has not been shown, probably due to the small distance between the sprayer needle and orifice. The small distance between the sprayer needle and orifice makes it difficult to position two sprayer needles such that both plumes are sampled by the source orifice but do not interfere with one another.
It would be advantageous to provide an electrospray ionization device with a simple design that has increased positional alignment tolerances and that is capable of both single and multiple nanoelectrospray ionization and standard electrospray ionization.
The present invention generally relates to electrospray ionization devices for performing standard electrospray ionization and nanoelectrospray ionization. An electrospray ionization device is provided that comprises one or more electrospray needles and a capillary. Each needle has a distal end for receiving a sample, a tip for spraying the sample in fluid communication with the distal end, and an electrical contact for contacting at least some portion of sample therein. The capillary has an inlet, an outlet, and an interior conduit in fluid communication with the inlet and the outlet, and is located in proximity to the tip or tips of the one or more electrospray needles. The inlet of the capillary has a counter-electrical contact, and at least a portion of the inlet has a larger inner diameter than any inner diameter of the interior conduit. The electrospray ionization device further comprises means for generating an electrical potential difference between the counter-electrical contact of the capillary inlet and the electrical contact(s) of the one or more electrospray needles.
In another arrangement, an electrospray ionization device is provided that comprises one or more electrospray needles and a capillary. Each needle has a distal end for receiving a sample, a tip for spraying the sample in fluid communication with the distal end, and an electrical contact for contacting at least some portion of sample therein. The capillary has an inlet, an outlet, and an interior conduit in fluid communication with the inlet and the outlet, and is located in proximity to the tip or tips of the one or more electrospray needles. The inlet of the capillary has a counter-electrical contact that comprises a high transmission metal mesh covering the inlet. The electrospray ionization device further includes means for generating an electrical potential difference between the counter-electrical contact of the capillary inlet and the electrical contact(s) of the one or more electrospray needles.
In yet another arrangement, an electrospray ionization device is provided that comprises one or more electrospray needles and a capillary. Each needle has a distal end for receiving a sample, a tip for spraying the sample in fluid communication with the distal end, and an electrical contact for contacting at least some portion of sample therein. The capillary has an inlet, an outlet, and an interior conduit in fluid communication with the inlet and the outlet, and is located in proximity to the tip or tips of the one or more electrospray needles. The inlet of the capillary is shaped to aerodynamically focus ions generated from the one or more electrospray needles into the interior conduit of the capillary, and at least a portion of the inlet has a larger inner diameter than any inner diameter of the interior conduit. The inlet also has a counter-electrical contact. The electrospray ionization device further comprises means for generating an electrical potential difference between the counter-electrical contact of the capillary inlet and the electrical contact(s) of the one or more electrospray needles. The capillary and the one or more electrospray needles are arranged such that, under electrospray conditions, at least a portion of ions generated from each of the one or more electrospray needles will enter the inlet of the capillary.
In a further arrangement, a nanoelectrospray ionization device is provided that comprises one or more nanoelectrospray needles and a capillary. Each of the one or more needles has a distal end for receiving a sample, a tip for spraying the sample in fluid communication with the distal end, and an electrical contact for contacting at least some portion of sample therein. The capillary has an inlet, an outlet, and an interior conduit in fluid communication with the inlet and the outlet, and is located in proximity to the tip or tips of the one or more nanoelectrospray needles. The inlet is shaped to aerodynamically focus ions generated from the one or more nanoelectrospray needles into the interior conduit of the capillary. At least a portion of the inlet has a larger inner diameter than any inner diameter of the interior conduit. The inlet also has a counter-electrical contact comprising a high transmission metal mesh covering the inlet of the capillary. The nanoelectrospray ionization device further comprises means for generating an electrical potential difference between the counter-electrical contact of the capillary inlet and the electrical contact(s) of the one or more nanoelectrospray needles.
In yet a further arrangement, an electrospray ionization device is provided that comprises one or more electrospray needles and an ion sampling device. Each needle has a distal end for receiving a sample, a tip for spraying the sample in fluid communication with the distal end, and an electrical contact for contacting at least some portion of sample therein. The ion sampling device has an entrance, an exit, and an interior in fluid communication with the entrance and the exit, and is located in proximity to the tip or tips of the one or more electrospray needles. The entrance defines an opening that has a larger area than an opening defined by the exit. The ion sampling device also has a counter-electrical contact. The electrospray ionization device further comprises means for generating an electrical potential difference between the counter-electrical contact and the electrical contact(s) of the one or more electrospray needles.
In another arrangement, an electrospray ionization device is provided that comprises one or more electrospray needles and means for sampling ions generated during electrospray ionization. Each of the one or more needles have a distal end for receiving a sample, a tip for spraying the sample in fluid communication with the distal end, and an electrical contact for contacting at least some portion of sample therein. The means for sampling ions is shaped to aerodynamically focus ions generated from the one or more electrospray needles and includes a counter-electrical contact. The electrospray ionization device further comprises means for generating an electrical potential difference between the counter-electrical contact and the electrical contact(s) of the one or more electrospray needles. The means for sampling ions and the one or more electrospray needles are arranged such that, under electrospray conditions, at least a portion of ions generated from each of the one or more electrospray needles will be sampled by the means for sampling ions.